Various types of testers for crystalline substances are known in the prior art and in the patent literature. These include apparatus for determining the genuineness of diamonds employing conductivity measurements. Such apparatus is exemplified in U.S. Pat. Nos. 4,255,962; 4,364,677; and 4,344,315 all of which employ temperature sensitive elements, such as thermistors, located at the tip of a probe, which tip is firmly contacted with a diamond to be tested. The thermistors measure the thermal response of the probe tip. Devices of the type described in the aforesaid U.S. Patents are relatively expensive, sensitive and complex electronically.